3,3-diphenylpropylamines which act as muscarinic receptor antagonists and are useful in the treatment of urinary incontinence and other symptoms of urinary bladder hyperactivity are known. Said compounds include N,N-diisopropyl-3-(2-hydroxy-5-methylphenyl)-3-phenylpropylamine, the (R) enantiomer of which is known as Tolterodine.
Another compound with a similar structure is 5-hydroxymethyl tolterodine, which is the main metabolite of Tolterodine (Nilvebrant et al. Pharmacol. Toxicol, 1997, 81(4), 169-172), a potent muscarinic receptor antagonist (WO 94/11337).

WO 99/058478 describes the therapeutic usefulness of phenolic esters of said main metabolite of Tolterodine, especially of isobutyric acid 2-((R)-3-N,N-diisopropylamino-1-phenylpropyl)-4-(hydroxymethyl)phenyl ester, known as Fesoterodine. Said document also describes the formation of their salts, particularly, the formation of Fesoterodine fumarate.

U.S. Pat. No. 5,559,269 discloses the preparation of 5-hydroxymethyl tolterodine and Fesoterodine through a very long synthesis process. The required chirality is introduced in step 3 by resolution of intermediate N,N′-diisopropyl-3-(2-benzyloxy-5-bromophenyl)-3-phenylpropyl amine (1) with tartaric acid.

An alternative process for preparing chiral intermediate (1) is described in WO 99/58478. This process comprises the asymmetric addition of phenyl magnesium bromide to a chiral α,β-unsaturated amide.

Compound (1) is obtained through long and cost-inefficient processes that require the use of expensive chiral reagents, which make it a non-suitable intermediate for the preparation of Fesoterodine and related compounds.
A different approach is described in EP 1289929 B1, by means of a synthetic route in which a coupling in acid medium is initially performed, forming a dihydrocoumarin as a racemic intermediate. Said intermediate is then subjected to a stereoselective resolution process to obtain the suitable enantiomer. The latter is subsequently reduced to a lactol derivative, in which a diisoalkylamine is introduced by means of a reductive amination. Although the process is shorter, many synthesis steps are still required. In addition, the use of the aluminum tert-butoxide as a reducing agent is a considerable problem of toxicity and added cost to the process.

WO 2007/138440 describes a route of synthesis through the formation of a dihydrocoumarin intermediate, by means of a reaction needing conditions of reflux in toluene and toluene/hydrochloric acid for long time periods and with a low yield.

Optical resolution is performed on compound N,N′-disiopropyl-3-(2-hydroxy-5-hydroxymethyl-phenyl)-3-phenylpropyl amine (2), which is the last intermediate of the synthesis, leading to more than 60% product loss at this point and, thus, making it a very expensive process. WO 2011/158257 refers to the optical resolution of compound (2) with D-(+)-maleic acid and consequently also has the same disadvantages.
US 2011/105783 and WO 2011/145019 refer to the resolution of intermediate N,N′-diisopropyl-3-(2-hydroxy-5-methylcarboxylate-phenyl)-3-phenylpropyl amine (3) with camphorsulfonic or dibenzoyltartaric acid.

In the reported cases, processes based on optical resolution through formation of diastereomeric salts do not typically give rise to the chiral salts in a suitable diastereomeric excess, making necessary to further purify the compound by subsequent recrystallizations.
Furthermore, an additional step to cleave the diastereomeric salt and recover the desired enantiomer, which is further transformed into the final product, is required.
In view of the above, it is still necessary to provide an alternative process for obtaining optically active intermediates for the preparation of Fesoterodine and related 3,3-diphenylpropylamines.
GB 948,583 discloses the resolution of racemic 7-methoxy-1,2,3,4,4a,9,10,10a-octahydro-phenanthrene-carboxylic acid by transformation into the corresponding acid chloride, treatment with L-menthol and separation of the resulting diastereomeric compounds.